1. Field of the Present Invention
This invention relates to a lithographic printing method and a printing press that can be used in implementing the method.
2. Description of the Related Art
A conventionally known ink supplying apparatus that uses emulsion ink is shown in FIG. 5. Being generally indicated by 50, the apparatus comprises a form roller 52 in contact with a plate cylinder 51, an adjuster roller 53 in contact with the form roller 52, a cooling mechanism (not shown) for cooling the adjuster roller 53, an ink fountain roller 54 in contact with the form roller 52, and an ink fountain 56 provided adjacent the ink fountain roller 54 to hold emulsion ink 55 (see JP 55-7453 A (the term xe2x80x9cJP XX-XXXXXX Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) at pages 1-4 and FIG. 3).
In the illustrated ink supplying apparatus 50, the ink fountain roller 54 draws the emulsion ink 55 from the ink fountain 56 and supplies it to the form roller 52. The emulsion ink on the form roller 52 is subjected to shear stress under the nip pressure between the adjuster roller 53 and the form roller 52 while at the same time it is cooled by the cooling mechanism. As the result, the ink undergoes emulsion""s disruption and separates into the ink and aqueous components. The resulting ink and aqueous components are transferred from the form roller 52 onto a lithographic printing plate Ps on the plate cylinder 51. The ink and aqueous components are then transferred from the lithographic printing plate Ps to a blanket cylinder 57 according to the image and non-image areas of the lithographic printing plate Ps. From the blanket cylinder 57, the ink and aqueous components are transferred to printing paper P as it is held between the blanket cylinder 57 and an impression cylinder 58, whereby the printing process is completed.
A problem with the above-described ink supplying apparatus is that as printing progresses, the proportion of the ink and aqueous components may vary in the ink fountain 56 and other parts, making it impossible to maintain consistent printing operations.
Specifically, if the consumption of the aqueous component is high, the concentration of the ink component will increase as printing proceeds, eventually increasing the chance for the occurrence of scumming. Conversely, if the consumption of the aqueous component is low, its concentration will increase as printing proceeds, causing such problems as lower image density and waterlogging due to over-emulsification.
In order to deal with these problems, it has been proposed that the moisture content of the emulsion ink be detected with a moisture sensor and that on the basis of the result of the detection, the emulsion ink be replenished with the ink component or the aqueous component to maintain the preferred moisture content (see JP 2001-514104 A).
However, the moisture content of the emulsion ink is difficult to measure with high precision, so it is difficult to control the emulsion ink such that it consistently has the preferred moisture content. As a further problem, the moisture sensor is expensive and contributes to a higher equipment cost.
An object of the present invention is to provide a method of emulsion-ink based lithographic printing that can maintain predetermined a proportion of an ink component and an aqueous component so as to realize consistent printing operations at low cost, and to provide an apparatus for use in implementing the method.
The present invention provides the following lithographic printing methods (1) and printing press (2).
(1) A lithographic printing method which performs lithographic printing with emulsion ink as it is supplied from an ink fountain which is a reservoir of the emulsion ink to a lithographic printing plate, comprising the steps of:
computing amounts of consumption of ink and aqueous components of the emulsion ink on the basis of a percent image area of the lithographic printing plate; and
replenishing the ink fountain with at least one member of the group consisting of the ink component, the aqueous component and the emulsion ink in accordance with the computed amounts of consumption of the ink and aqueous components.
(2) A printing press which performs printing using emulsion ink, comprising:
an emulsion ink supplier having an ink fountain which is a reservoir of the emulsion ink and a form roller which supplies the emulsion ink to a lithographic printing plate on a plate cylinder; and
a replenisher having a replenishment control unit which determines amount of replenishment in which the ink fountain is to be replenished with at least one member of the group consisting of the ink component, the aqueous component and the emulsion ink in accordance with amount of consumption of each of the ink and aqueous components of the emulsion ink as computed on the basis of percent image area of the lithographic printing plate and a replenishing unit which replenishes the ink fountain with at least one member of the group consisting of the ink component, the aqueous component and the emulsion ink in accordance with the determined amount of replenishment.
According to the present invention, the problems that accompany the process of lithographic printing with emulsion ink, such as scumming which results from high consumption of the aqueous component, as well as faint image density, waterlogging due to over-emulsification and the like which result from low consumption of the aqueous component can be prevented and one can produce high-quality printed matter that is free from any deterioration in image quality on account of those problems.